An overlooked consequence of dietary mixing: a varied diet reduces interindividual variance in fitness

The determinants of diet breadth are of interest to nutritionists, ecologists and evolutionary biologists. A recent synthesis addressing this issue found conflicting evidence for the relationship between diet breadth and mean individual fitness. Specifically, that analysis found that whilst on average a mixed diet does increase mean fitness, in some instances a single food provides equal (or higher) fitness than a mixed diet. Critical to ecological and evolutionary considerations of diet, however, is not only average, but also variance in fitness. We combine contemporary meta-analytic methods with models of nutritional geometry to evaluate how diet affects between-individual variance in fitness within generalist consumers from a range of trophic levels. As predicted by nutritional geometry, we found that between-individual variance in fitness-related traits is higher on single-food than mixed-food diets. The effect was strong for longevity (57% higher) and reproductive traits (37%), and...